ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus GmbHGöttingen, Germany10.5194/acp-10-8783-2010Observations of OH and HO<sub>2</sub> radicals over West AfricaCommaneR.14FloquetC. F. A.15InghamT.12StoneD.13EvansM. J.3HeardD. E.121School of Chemistry, University of Leeds, LS2 9JT, UK2National Centre for Atmospheric Science, University of Leeds, LS2 9JT, UK3Institute for Climate & Atmospheric Science, School of Earth & Environment, University of Leeds, LS2 9JT, UK4now at: School of Engineering & Applied Sciences, Harvard University, Cambridge, USA5now at: National Oceanography Centre, University of Southampton, Southampton, UK17092010101887838801This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from http://www.atmos-chem-phys.net/10/8783/2010/acp-10-8783-2010.htmlThe full text article is available as a PDF file from http://www.atmos-chem-phys.net/10/8783/2010/acp-10-8783-2010.pdf

The hydroxyl radical (OH) plays a key role in the oxidation of trace gases in
the troposphere. However, observations of OH and the closely related
hydroperoxy radical (HO<sub>2</sub>) have been sparse, especially in the tropics.
Based on a low-pressure laser-induced fluorescence technique (FAGE –
Fluorescence Assay by Gas Expansion), an instrument has been developed to
measure OH and HO<sub>2</sub> aboard the Facility for Airborne Atmospheric
Measurement (FAAM) BAe-146 research aircraft. During the African Monsoon
Multidisciplinary Analyses (AMMA) campaign, observations of OH and HO<sub>2</sub>
(HO<sub>x</sub>) were made in the boundary layer and free troposphere over West
Africa on 13 flights during July and August 2006. Mixing ratios of both OH
and HO<sub>2</sub> were found to be highly variable, but followed a diurnal cycle:
OH varied from 1.3 pptv to below the instrumental limit of detection, with
a median mixing ratio of 0.17 pptv. HO<sub>2</sub> varied from 42.7 pptv to
below the limit of detection, with a median mixing ratio of 8.0 pptv. A
median HO<sub>2</sub>/OH ratio of 95 was observed. Daytime OH observations were
compared with the primary production rate of OH from ozone photolysis in the
presence of water vapour. Daytime HO<sub>2</sub> observations were generally
reproduced by a simple steady-state HO<sub>x</sub> calculation, where HO<sub>x</sub> was assumed to be formed from the primary production of OH and lost
through HO<sub>2</sub> self-reaction. Deviations between the observations and this
simple model were found to be grouped into a number of specific cases: (a)
within cloud, (b) in the presence of high levels of isoprene in the boundary
layer and (c) within a biomass burning plume. HO<sub>2</sub> was sampled in and
around cloud, with significant short-lived reductions of HO<sub>2</sub> observed.
Up to 9 pptv of HO<sub>2</sub> was observed at night, with HO<sub>2</sub> above 6 pptv
observed at altitudes above 6 km. In the forested boundary layer, HO<sub>2</sub>
was underestimated by a steady state calculation at altitudes below
500 m but overestimated between 500 m and 2 km. In a biomass burning
plume, observed HO<sub>2</sub> concentrations were significantly below those
calculated.